The present invention relates generally to an inverter for an electric traction motor, and more particularly is an improved inverter with a heat sink that utilizes oil (hydraulic fluid) as the heat transfer fluid.
Traction vehicles, such as locomotives or high power off-road vehicles, are driven by electrically powered traction motors which drive the wheels of the vehicle. The traction motors operate on AC power, but the power generated by the motor of the vehicle is DC. The DC potential generated by the motor must therefore be converted from DC to AC current in an inverter. The electric power generation/inversion requires the use of multiple semiconductor devices, and switches to control the semiconductor devices, all of which generate a great deal of heat. To dissipate the heat produced in the inverter, current art vehicles use either water or air cooling systems, or both in combination. These current art methods of cooling lead to several problems.
For any device to be air cooled, there must be adequate space around the device for air to flow in sufficient volume to remove the heat. Since traction motor applications typically utilize three-phase AC current, six IGBT (insulated gate bipolar transistor) switches must be employed. The power requirements of the motors require that a capacitor bank be present in the inverter, along with the accompanying sensors, etc. The number of components required mandates a significant space requirement which is greatly exaggerated due to the need for space to accommodate air flow around the inverter.
In direct contradiction to the need for open space for cooling air flow is the fact that electrical devices function best in enclosed, non-ventilated environments. This kind of environment reduces the potential of contaminant buildup. Contaminant buildup can not only impede the desired heat transfer, but may also cause an electrical failure of the device. Therefore air cooling directly creates a situation detrimental to the function of the electrical device, in this case an inverter.
Because of the problems caused by air cooling, some current art devices utilize water cooling in the inverter. Water cooled inverters can operate in a more controlled environment, but the water cooling system is generally not readily available. Motors in vehicles of the class which is the subject of this invention, those that utilize electrically powered traction wheels, are usually oil cooled. Thus, utilization of an inverter that requires water cooling leads to the necessity of including a water cooling system in a motor that would not otherwise have it. Still more space is therefore required.
Because of the size requirements demanded by the cooling systems of current art inverters, the inverter comprises a large unit contained in a compartment dedicated only to the inverter. This necessitates that lead wires for control and feedback systems must be fairly long, typically anywhere from 2 to 10 feet. Longer wires are by necessity heavier than shorter wires, both in terms of weight and electrical rating. Longer wires significantly increase the potential for distorted signals.
Accordingly, it is an object of the present invention to provide an inverter for an electric traction motor that utilizes the same cooling system used for the motor.
It is a further object of the present invention to provide an inverter that has significantly reduced space requirements.
It is a still further object of the present invention to provide an inverter that can be formed with generally planar components, thereby reducing inductance of the inverter.
The present invention is an oil-cooled inverter for an electric traction motor. The inverter comprises six IGBT switches to generate 3-phase AC electrical current as its output. The switches are mounted on heat sinks that use hot motor oil (hydraulic fluid) as the heat transfer fluid. An interior of each of the heat sinks contains a heat transfer medium. In the preferred embodiment, the heat transfer medium is a plurality of silver plated copper balls. The copper balls are brazed to each other and to the walls of the heat sinks in an assembly process. The heat sink housing is made of molybdenum, which has a thermal expansion rate equal to that of silicon. This allows the circuitry of the inverter to be mounted directly on the top surface of the heat sinks, as the expansion and contraction of the heat sink housing will match that of the silicon substrate of the chips. Since both elements, the heat sink housing and the chip substrate, expand and contract at an equal rate during heating and cooling, the problem of the chip substrate cracking and breaking due to thermal flexing is eliminated.
The IGBT switch/heat sink assemblies are mounted in a planar fashion in a laminated bus device. The bus device also houses a bank of capacitors for the inverter. The structure of the bus device allows the IGBT switches and the capacitors of the inverter to be assembled as generally planar elements, thereby reducing inductance of the device. This allows the switches to function more efficiently.
The inverter of the present invention is in most ways electrically equivalent to prior art inverters. One key exception is that the upper surface formed by the bank of heat sinks is electrically hot, and is used as part of the circuitry. This allows the elimination of the insulating plate on which the inverter circuitry is normally mounted, as the chips of the inverter circuitry of the present invention are mounted directly on the upper surface of the heat sinks of the inverter.
One of the chief advantages of the present invention is that it greatly reduces the space required for the inverter in the engine compartment of the vehicle. The inverter module of the present invention requires only 10% the amount of space required by the inverter module of the inventors"" current art. The weight of the inverter module of the present invention is also greatly reduced, being only 20% of that of the current art. This tremendous reduction in size and weight leads to the added benefit of making the inverter module far less expensive and more practical to xe2x80x9csparexe2x80x9d, that is, to stock an entire inverter module for repair use.
Another advantage of the present invention is that the reduced size and compact assembly of the inverter allows it to be mounted directly to the motor (generator). This significantly reduces the length of the lead wires of the feedback and control systems.
A still further advantage of the present invention is that it utilizes the same coolant supply system as does the electric motor of the vehicle, thereby eliminating the necessity of a water cooling system.
Yet another advantage of the inverter of the present invention is that the chips of the inverter circuitry are mounted directly on the surface of the heat sink, thereby eliminating components and space requirements. The surface of the heat sinks is electrically hot, and serves as part of the circuitry.
Still another advantage of the present invention is that the planar structure reduces inductance in the inverter.
These and other objects and advantages of the present invention will become apparent to those skilled in the art in view of the description of the best presently known mode of carrying out the invention as described herein and as illustrated in the drawings.